AU2008238310B2

AU2008238310B2 - Method for the estimation of fluids moved in compartmented areas of the subsoil

Info

Publication number: AU2008238310B2
Application number: AU2008238310A
Authority: AU
Inventors: Massimo Antonelli; Salvatore Giammetti; Italiano Giori; Luca Savini; Luigi Terzi
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2007-04-13
Filing date: 2008-03-28
Publication date: 2013-03-14
Anticipated expiration: 2028-03-28
Also published as: US20100083753A1; US8326541B2; BRPI0810217A2; WO2008125226A2; EA019193B1; CA2683904A1; WO2008125226A3; EA200901302A1; ITMI20070746A1; AU2008238310A1; EG26290A; CN101680966B; CN101680966A; EP2135118A2

Abstract

The present invention relates to a method for estimating fluid volumes moved in compartmented areas of the subsoil, including steps which consist in effecting, in a survey field, a series of gradiometric measurement campaigns "for framing/calibrating the area" at pre- established time intervals, for each of which the relative variation ΔV

Description

WO 2008/125226 PCT/EP2008/002659 5 METHOD FOR THE ESTIMATION OF FLUIDS MOVED IN COMPART MENTED AREAS OF THE SUBSOIL The present invention relates to method for estimating 10 volumes of fluids moved in compartmented areas of the subsoil, such as, for example, natural deposits, also called reservoirs, it can be applied industrially in oil fields, and additionally, also for monitoring the produc tion and storage of hydrocarbons and reducing mining 15 risks during the explorative phase and development of on shore and off-shore reservoirs. In the exploration of subsoil resources, it is custom ary to rely on the measurement of the vertical component of the gravity field and the vertical gradient of the 20 same field. From an analysis of these data, it is possi ble to deduce information on the density distribution of the subsoil which characterizes a particular site. On the basis of this methodology, it is also possible to obtain the mass variation of the hydrocarbons present 25 inside a reservoir. The idea at the basis of this technique is that as the -1- WO 2008/125226 PCT/EP2008/002659 movements of hydrocarbons inside a reservoir are corre lated to density variations, they can be appreciated by means of vertical gravity gradient measurements. It is since the thirties', in fact, that measurements 5 of the gravimetric field gradient have been successfully used in the exploration of resources of the subsoil. Since 1936 the importance has been known of the use of the vertical gradient which, as it has a better resolu tion and is relatively insensitive to regional effects, 10 often has particular structures which cannot be easily obtained from gravimetric field data. The measurement of the vertical gravimetric field gra dient can be effected by means of specific instruments called gradiometers. 15 Alternatively, the vertical gradient of the gravity field of a point can be measured, with good approxima tion, by means of the almost contemporaneous acquisition of two gravimetric measurements referring to different heights. 20 In this second case, before interpreting the data ac quired in the field in geological terms, their reduction in terms of Bouguer anomaly is frequent, from which the undesired effects are removed and the calculation and analysis of the vertical gradient is subsequently ef 25 fected. -2- WO 2008/125226 PCT/EP2008/002659 The most important corrections to be made are the fol lowing: - Instrumental drift - Tidal correction 5 - Latitude correction - Free Air correction - Bouguer correction - Topographic correction The description of these corrections is treated 10 hereunder. Instrumental drift: the readings of data with a gravime ter undergo time variations due to the elastic character istics of the materials which form the instrument itself. The instrumental drift can be easily determined by re 15 peating the measurement in the same station in different times, typically every 1-2 hours. The representation re ferring to Cartesian axes gives the drift curve which, for many gravimeters is of the linear type. A definite value is subtracted with each measurement 20 effected in subsequent stations, on the basis of the measurement time. Tidal correction: the drift measured in reality contains the further contribution of an effect of the sea type due to moon-sun attraction (tide). The correction to be made 25 is calculated on a theoretical basis by means of formulae -3- WO 2008/125226 PCT/EP2008/002659 which allow the quantification of this effect, such as, for example, the Longman formula. Latitude correction: both the Earth's rotation and its equatorial swelling produce an increase in gravity with 5 the latitude, and this must be considered when reducing the gravity data observed. Free air correction: this is a correction used in order to consider the altitude of the measuring station. Bouguer correction: this correction is used to consider 10 the attraction due to the interposed masses between the measuring station and the reference surface. In 1749, Bouguer suggested that this additional attraction could have been calculated like that due to the action of an infinite horizontal plate having a thickness equal to the 15 elevation from sea level of the measuring station. Topographic correction: The approximation of the plate may be unsatisfactory in an area with an articulated to pographic trend. Under these conditions, it is appropriate to add a 20 correction in order to consider the masses above the plate and those whose contribution has been erroneously subtracted in the Bouguer correction. After the reductions listed, the vertical gradient of the field is calculated as described hereunder. 25 Variations in density in the subsoil with time can be -4measured and monitored from the measurements of the vertical gravity field gradient. This method is already in use for measuring and monitoring water layers and geothermic fields. 5 A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission or a suggestion that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. 10 An aspect of the present invention is to provide a method for the estimation of fluid volumes, such as, for example, liquid and/or gaseous hydrocarbons moved in compartmented areas of the underground, following, for example, production, injection, and/or storing. 15 Within the above mentioned aspect, it is necessary to draw a calibration curve characteristic for each single field reservoir. According to an aspect of the present invention there is provided a method for estimating fluid volumes moved in 20 compartmented areas of the subsoil, including the following steps which consist in: a) effecting, in a survey field, a series of gradiometric measurement campaigns at pre-established time intervals, for each of which the relative variation AVi of the fluid volumes contained in a corresponding compartmented -5area of the subsoil is known; b) on the basis of the measurement effected, calculating, for each measurement campaign subsequent to the former one, a parameter Pii correlated to the entire survey area; c) on the basis of the 5 known fluid volume variations AVi and the relative parameter Pii calculated, determining by approximation a variability law which relates the parameter Pli with the volume variation of the fluids; d) estimating the fluid volumes moved in said compartmented area, on the basis of any measurement campaign 10 subsequent to those defined under step a) and the law of variability determined in step c). The characteristics and advantages of an estimation method of the fluid volumes in compartmented areas, according to the present invention, will appear more evident from the following 15 illustrative and non-imitating description. The estimation method of fluid volumes in the sub- soil, according to the invention, comprises a first gra -Sa- WO 2008/125226 PCT/EP2008/002659 diometric measurement phase "to frame/calibrate the area" on the basis of which a model is determined, by statisti cal approximation, for quantifying the movement of the fluid volumes, such as, for example, hydrocarbons, pro 5 duced, injected and/or stored in the subsoil. More specifically, a series of measurement campaigns are effected, repeated over a certain period of time, with the aim of quantifying the amount of the fluid move ments, by means of a model of their production, injection 10 and/or storage. The degree of accuracy of this model is then evalu ated through the statistical analysis of the data ob tained. The measurement operation campaigns are effected on a 15 series of stations suitably dislocated in the survey area. The measurement is effected, for each single station, using a precision gravimeter and a tripod suitably posi tioned to allow measurement at a certain distance and 20 constant from the ground. For the geophysical applications mentioned above, it is advisable to use gravimeters having a precision not lower than pGal (microgal). From an operative point of view, the campaign meas 25 urements consist of the following phases. -6- WO 2008/125226 PCT/EP2008/002659 A first ground gravimetric measurement is acquired Gbot followed by a second gravimetric measurement Gtop by positioning the gravimeter at a distance dh from the ground using a tripod. 5 The distance dh from the ground is preferably kept constant in each single station and during the whole sur vey measurement. The Applicant has in fact observed that, by maintaining the distance from the ground constant, in each single station and for the whole survey, the meas 10 urements and definitions of gradiometric anomalies are more accurate. On the basis of the Gbot and Gtop values measured, the gravity values G*bot, G*top are then determined, corrected with respect to the effects previously discussed. 15 The vertical gradient value of the gravimetric field is obtained from the correct gravity values, by means of the following formula: (G*bot - G*top) VGG = __ (1) dh 20 The determination of the difference in level dh between two relative measurements in each single station is ef fected by means of a laser distantiometer with millimet ric precision. This acquisition and elaboration procedure is then 25 repeated with time on the same area, insisting on the -7- WO 2008/125226 PCT/EP2008/002659 same measurement stations. A time lapse signal is then obtained, i.e. a signal linked to the gravity variations in the subsoil over a period of time. 5 The time lapse signal is calculated as the difference between two gradiometric surveys effected in the same measurement station, at different times: TL ji = VGGi - VGGj (2) In this way it is possible to evaluate which zones of 10 the area examined have undergone a density variation, thus revealing where there have been relative movements of fluids. In this way, a qualitative result is obtained, relat ing to the volume variations of fluids in the subsoil, 15 connected, from case to case, with production, injection and/or storage operations. The Applicant subsequently identified a calibration curve characteristic of each single storage or production reservoir, which correlates the variations in the gradi 20 ometric values detected over a certain time lapse, with the corresponding volumes of the fluids moved, or pro duced, injected and/or stored, within the same time pe riod. The Applicant has also developed, tested and validated 25 an assessment method of said calibration curve. -8- WO 2008/125226 PCT/EP2008/002659 As far as qualitative evaluations of the movements of masses with time are concerned, the Applicant deemed it suitable to find a calibration curve which would relate volumetric variations of fluids with time lapse data and 5 not with the gravity gradient data of a single assess ment. For this purpose, it was necessary to assess a charac teristic parameter of the fluid volume variation associ ated with the production, injection and/or storage opera 10 tions. The use of the integral of time lapse values on the whole definition dominium of the area examined, proved to be adequate for this purpose: 15 JJTL1(x,y)dxdy (3a) The value so obtained is preferably normalized with respect to the integral of the first time lapse TL 12 , thus assuming the first assessment as reference point. An adimensional parameter Pii is thus obtained, bound 20 to the quantitative global variation of the volumes, de fined as JJ TL 1 i(x,y)dxdy I JJTL1 2 (x,y)dxdy (4a) 25 The parameter Pii can be calculated on the basis of -9- WO 2008/125226 PCT/EP2008/002659 the values acquired with the measurement campaign number i. In the same way, it is also suitable to use as parame ter Pii the summation of the time lapse values measured 5 in the n measurement stations distributed on the whole survey area: X TL 1 i (n) (3b) n Also in this case, the value thus obtained is prefera bly normalized with respect to the summation of the first 10 time lapse measurements TL 12 in order to obtain an adi mensional parameter: 1, TL 1 i (n) (4b) n Pi= 15 XTL 12 (n) n The Applicant then constructed the calibration curve on the basis of at least three acquisition campaigns, on known volumes of fluids moved (produced, injected and/or 20 stored). On the basis of the parameters Pii associated with the time lapse whose moved'volumes AVi are known, the rela tionship between the parameters P 1 i and the corresponding known volumes produced, injected and/or stored AVi was 25 reconstructed by approximation. -10- WO 2008/125226 PCT/EP2008/002659 Various controls of the results obtained demonstrated and confirmed that the variation law thus determined al lows the volume variation of moved fluids (produced, in jected and/or stored) to be estimated starting from gra 5 diometric measurement campaigns subsequent to those of the first phase "for framing/calibrating the area". In a purely illustrative manner, a method is described herein for the determination of a possible law which cor relates the gradiometric values measured with the volume 10 of fluids, such as, for example, hydrocarbons, moved i.e. produced, injected and/or stored. Assuming a linear relationship between the parameter

P

1 i and the volume of hydrocarbons moved AVi, of the type: P=a+b AV (5) 15 a line can be constructed to be used for estimating AV starting from the fourth survey onwards. The Pli parameters associated with the first three measurement campaigns and a knowledge of the relative volumes moved are necessary for defining the parameters 20 a and b. For the evaluation of the parameters a and b, with the relative uncertainties 5a and 5b, it is possible, for example, to use the approximation method of the square minima, which minimizes the differences between the theo 25 retical values of the ideal straight line and the data -11- WO 2008/125226 PCT/EP2008/002659 observed, i.e. the following expression: n (Pi - a - bAVi) 2 X2 X (6) 5 i=1 P Once a and b have been determined in this way, it is possible, knowing the value of the parameter Pli, to evaluate the corresponding value of AV. The uncertainty and consequently the sensitivity of 10 the method depend on the uncertainty of the parameters a and b and the uncertainty 6P on the parameter P which comes from the gravity measurements. The equation for estimating the moved volumes is: (P-a) AV= (7) 15 b Therefore, the associated error SAV is given by: DAV aAV aAV (8) 6AV= 6P + 6a + - b 20 BP Ba 8b In the same way it is possible to assume a polynomial variability law of a suitable degree and use, as an ap proximation method, the square minima method or interpo 25 lation. -12- WO 2008/125226 PCT/EP2008/002659 The characteristics of the method object of the pre sent invention, as also the relative advantages, are evi dent from the above description. The Applicant has added an algorithm to the determina 5 tion of the vertical gravity gradient and relative time lapse processings, for estimating the fluid volumes moved in reservoirs over a period of time. Finally, it is evident that the method thus conceived can undergo further modifications and variations, all in 10 cluded in the scope of the invention. 15 20 25 -13-

Claims

1. A method for estimating fluid volumes moved in 5 compartmented areas of the subsoil, including the following steps which consist in: a) effecting, in a survey field, a series of gradiometric measurement campaigns at pre-established time intervals, for each of which the relative variation AVi of the 10 fluid volumes contained in a corresponding compartmented area of the subsoil is known; b) on the basis of the measurement effected, calculating, for each measurement campaign subsequent to the former one, a parameter Pii correlated to the entire survey 15 area; c) on the basis of the known fluid volume variations AVi and the relative parameter Pli calculated, determining by approximation a variability law which relates the parameter Pii with the volume variation of the fluids; 20 d) estimating the fluid volumes moved in said compartmented area, on the basis of any measurement campaign subsequent to those defined under step a) and the law of variability determined in step c). -14-

2. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 1, wherein the time lapse TL 1 j values are calculated from said gradiometric measurements effected at pre-established time 5 intervals of said phase a), as the difference between two gradiometric measurements VGGi, VGGj, effected in each measurement station at different times.

3. The method for estimating fluid volumes moved in 10 compartmented areas of the subsoil according to claim 2, wherein said parameter Pui is calculated on the basis of the integral of the time lapse TLii values on the whole definition range of the survey area. 15

4. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 3, wherein said parameter P1, is the integral of the time lapse TLuj values on the whole definition range of the survey area, normalized with respect to the integral of the first time lapse 20 TLuj.

5. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 2, wherein said parameter Pii is calculated on the basis of the 25 addition of the time lapse TL 11 values detected in the n -15- measurement stations distributed over the whole definition range of the survey area.

6. The method for estimating fluid volumes moved in 5 compartmented areas of the subsoil according to claim 5, wherein said parameter Pli is the addition of the time lapse TLii values detected in the n measurement stations distributed over the whole definition range of the survey area normalized with respect to the summation of the first time lapse TLii. 10

7. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 4 or 6, wherein said phase c) consists in determining a linear variability law between said calculated Pii parameters and the 15 corresponding known volume variations AVi.

8. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 4 or 6, wherein said phase c) consists in determining a polynomial 20 variability law between said calculated Pli parameters and the corresponding known volume variations AVi.

9. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 7 or 8, -16- wherein said variability law is determined by approximation through the square minima method or interpolation.

10. The method for estimating fluid volumes moved in 5 compartmented areas of the subsoil according to any one of the previous claims, wherein said series of gradiometric measurement campaigns in said phase a) are at least three.

11. The method for estimating fluid volumes moved in 10 compartmented areas of the subsoil according to any of the previous claims, wherein said phase a) for gradiometric measurements is effected, for each measurement station, by means of a gravimeter and a tripod, in order to allow the almost contemporaneous acquisition of two gravimetric 15 measurements Gbot and Geop referring to different elevations.

12. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 11, wherein gravimetric measurements G*bot and G*o corrected from a 20 series of influencing factors, are derived from said gravimetric measurements Gbot and Geop.

13. The method for estimating fluid volumes moved in compartmented areas of the subsoil according to claim 12, 25 further including determining a gradiometric measurement based -17- on corrected gravimetric measurements G*bot and G*top according to the following formula: (G'bot - G*top) VGG=._- - (1) dh 5 wherein VGG is the vertical gradient value of the gravimetric fluid and dh is the difference in level between two measurements relating to a single station.

14. The method for estimating fluid volumes moved in 10 compartmented areas of the subsoil according to claim 13, wherein the difference in level dh is effected by means of a laser distantiometer.

15. The method for estimating fluid volumes moved in 15 compartmented areas of the subsoil according to claim 13 or 14, wherein said difference in level dh must be kept constant for all the measurement stations of the entire survey.

16. The method for estimating fluid volumes moved in 20 compartmented areas of the subsoil according to any one of the claims from 1 to 10, wherein said gradiometric measurement campaign is effected, in each measurement station, by means of a gradiometer. -18-

17. A method for estimating fluid volumes moved in compartmented areas of the subsoil substantially as hereinbefore described. -19-